SonexBuilders.net

Probable cause released today.

http://www.ntsb.gov/aviationquery/brief ... 1705&key=1

Primary cause was fuel exhaustion followed by maneuvers that over stressed the airframe . Contributing factor was the kit manufacturer used a design guideline intended only for conventional tails.

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Too bad the GPS track was lost. I'd like to know if the failure was due to overspeed or some high G maneuver. We will never know. I do know that the fix looks like it will not allow this failure to occur again.

It bothers me that bad build technique, a pilot with known medical issues, and an airplane without fuel were not found to be part of the cause.....

Robbie
Sonex 1517

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To me it seems clear that it was a negative over G maneuver probably with some rudder thrown in:

The profile was transversely fractured, separating the horizontal and vertical flanges. Fracture features such as shear lips and rough matte grey surfaces indicated tensile and shear overstress fracturing. There were no indications of fatigue or other progressive crack growth. The profile fracture exhibited a 2.5 to 3 inches long tensile overstress in the vertical flange. The tensile region was at the right edge of the vertical flange, near the profile's fillet. The rest of the profile was fractured by shearing overstress in the horizontal flange.

The horizontal flange of the profile showed out-of-plane deformation, along with clear longitudinal elongations of the bolt holes. The vertical portion of the profile did not show any out-of-plane or within-plane deformation, but the two rightmost bolt holes exhibited elongation in the vertical direction.

Had the holes in the bulkhead that the forward angle attached to been better, it may have given enough margin to prevent the accident. With the low experience, and the fact that he ran out of fuel and then apparently did a big pushover, would he have survived if the tail stayed on? I am not sure. I think the motivation for Sonex to re-analyze this area is one positive thing to come out of the accident. There will be a lot more margin for poor workmanship and other factors.

Folks had mentioned the comfort they had in the tank being intact. The loads were probably low because it was empty. If it was full or partly full and survived that crash that would be a lot more telling. Did Sonex drop test the tank? I thought I read that somewhere.

Between the probable cause, the full narrative, the report, and the service bulliten I feel good about the Y tail. This is just my opinion of course. I never worked in accident investigation in my old company though I did work test.

Sonex1517 wrote:It bothers me that bad build technique, a pilot with known medical issues, and an airplane without fuel were not found to be part of the cause.....

Robbie
Sonex 1517

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Fuel exhaustion was found to be the primary cause.

fastj22 wrote:

Sonex1517 wrote:It bothers me that bad build technique, a pilot with known medical issues, and an airplane without fuel were not found to be part of the cause.....

Robbie
Sonex 1517

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Fuel exhaustion was found to be the primary cause.

Huh? Quoting the report -

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The in-flight failure of the aircraft’s Y-tail attachment structure during maneuvering flight due to overload. Contributing to the accident was, the kit manufacturer’s use of 14 CFR Part 23, Appendix A design guidelines intended for a conventional tail airplane without a V- (or Y-) tail.

Did you read something I missed?

Robbie
Sonex 1517

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From the prob cause:

It is likely that the pilot, distracted by the loss of engine power, allowed the airplane to enter an unusual attitude, and the tail separated during the pilot’s attempted recovery from the unusual attitude.

Running out of fuel would not be the cause of the tail coming off though. If I ever lose power(again) I don't think I would pull the tail off.

Thanks! I have no clue how to read thses things yet...


Robbie
Sonex 1517

Sent from my iPad using Tapatalk HD

fastj22 wrote:Probable cause released today.

http://www.ntsb.gov/aviationquery/brief ... 1705&key=1

Primary cause was fuel exhaustion followed by maneuvers that over stressed the airframe . Contributing factor was the kit manufacturer used a design guideline intended only for conventional tails.

So those "less than optimal building" findings shown in the pictures on the first page had nothing to do with the in flight breakup?